Automated test equipment (ATE) for digital integrated circuits are required to provide a stimulus to the integrated circuit (IC) and to measure the resultant digital response from the IC. Depending upon the size and function of the IC being tested, the power required for testing common ICs may range from less than one watt to greater than 50 watts. In order to meet the wide range of current and voltages required by various ICs, it is desirable that a power supply be programmable.
Since a power supply must be capable of meeting the current requirements for large ICs, it is also desirable that a power supply provide a means for current limiting in order to protect the test equipment and the circuit being tested.
FIG. 1A shows a conventional crowbar current limit scheme. When the load current reaches a specified limit, the power supply is switched off, with the load voltage and current being forced to zero. The power supply usually requires a reset to restore operation. This is a straightforward and cost effective current limiting technique to implement.
FIG. 1B shows a constant current limiting scheme. The constant current scheme allows for continued operation of the device being tested at the set maximum current; however, the power supply may be required to sustain a large voltage drop across its pass device, resulting in a large power dissipation by the supply. The requirement for handling the thermal load increases the cost and size of the power supply.
FIG. 1C shows a foldback technique that is a tradeoff between the crowbar and straight current limiting solutions. Instead of shutting off the supply current or maintaining a fixed value, the supply current is reduced in response to a drop in the load voltage when the current limit is reached. Although operation can be maintained at a reduced current, foldback limiting can have difficulty recovering from a short circuit, with the output voltage being limited if the load current rises above Ifb.
FIG. 2A shows an example of a conventional current limiting circuit 200. The load current is sensed by an instrumentation amplifier 205 by measuring the voltage drop across Rsense. The output of the instrumentation amplifier 205 is fed back into an error amplifier 210 that senses the output voltage Vout and compares it against a reference voltage Vref. It is desirable that the voltage and current sense loops be fast in order to guarantee fast transient response.
FIG. 2B shows an implementation of a foldback current limiting scheme 220. A Darlington pair pass device 225 includes transistors Q1 and Q2. A sensing network 230 comprises resistors R3, R4, R5, and PNP transistor Q3. Limiting is provided when increasing load current eventually turns on Q3, producing an increasing voltage drop across R6 that gradually turns off the pass device 225. The scheme 220 is dependent upon the base-emitter voltage of Q3, and thus is dependent upon the transistor fabrication variability. The current limit cannot be easily adjusted without circuit modification.
FIG. 3A shows an example of a low ripple power supply 300 that is generally used as a device power supply (DPS) in Automated Test Equipment (ATE) systems. In spite of the relatively low efficiency of linear voltage regulators, they are preferred for use as a low ripple regulator 305 due to the absence of switching noise. The dissipation in the regulator 305 is the product of the voltage difference (Vpwr−Vout) and the load current. A digital-to-analog converter (DAC) 310 may be used to set the output voltage.
FIG. 3B shows a DPS 340 similar to that of FIG. 3A with a high efficiency switching supply 345 used to provide a fixed Vpwr for a low ripple regulator 305. This scheme provides a stable input voltage for the linear voltage regulator 305; however, for low Vset efficiency is reduced by the increased voltage drop across the regulator 305. This problem is exacerbated when a low voltage part requires more current than its higher voltage counterpart, which is typically the case.